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    <title>Daytime.6 - An asynchronous UDP daytime server</title>
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            <div><h3 class="title">
                <a name="asio.tutorial.tutdaytime6"></a><a class="link" href="tutdaytime6.html"
                                                           title="Daytime.6 - An asynchronous UDP daytime server">Daytime.6
                - An asynchronous
                UDP daytime server</a>
            </h3></div>
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    </div>
    <h5>
        <a name="asio.tutorial.tutdaytime6.h0"></a>
        <span><a name="asio.tutorial.tutdaytime6.the_main___function"></a></span><a class="link"
                                                                                    href="tutdaytime6.html#asio.tutorial.tutdaytime6.the_main___function">The
        main() function</a>
    </h5>
    <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span
            class="special">()</span>
<span class="special">{</span>
  <span class="keyword">try</span>
  <span class="special">{</span>
</pre>
    <p>
        Create a server object to accept incoming client requests, and run the <a class="link"
                                                                                  href="../reference/io_context.html"
                                                                                  title="io_context">io_context</a>
        object.
    </p>
    <pre class="programlisting">    <span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">io_context</span> <span class="identifier">io_context</span><span
            class="special">;</span>
    <span class="identifier">udp_server</span> <span class="identifier">server</span><span class="special">(</span><span
                class="identifier">io_context</span><span class="special">);</span>
    <span class="identifier">io_context</span><span class="special">.</span><span class="identifier">run</span><span
                class="special">();</span>
  <span class="special">}</span>
  <span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span
                class="special">::</span><span class="identifier">exception</span><span
                class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span
                class="identifier">what</span><span class="special">()</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
  <span class="special">}</span>

  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
    <h5>
        <a name="asio.tutorial.tutdaytime6.h1"></a>
        <span><a name="asio.tutorial.tutdaytime6.the_udp_server_class"></a></span><a class="link"
                                                                                     href="tutdaytime6.html#asio.tutorial.tutdaytime6.the_udp_server_class">The
        udp_server class</a>
    </h5>
    <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">udp_server</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
    <p>
        The constructor initialises a socket to listen on UDP port 13.
    </p>
    <pre class="programlisting">  <span class="identifier">udp_server</span><span class="special">(</span><span
            class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span
            class="special">&amp;</span> <span class="identifier">io_context</span><span class="special">)</span>
    <span class="special">:</span> <span class="identifier">socket_</span><span class="special">(</span><span
                class="identifier">io_context</span><span class="special">,</span> <span
                class="identifier">udp</span><span class="special">::</span><span
                class="identifier">endpoint</span><span class="special">(</span><span class="identifier">udp</span><span
                class="special">::</span><span class="identifier">v4</span><span class="special">(),</span> <span
                class="number">13</span><span class="special">))</span>
  <span class="special">{</span>
    <span class="identifier">start_receive</span><span class="special">();</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="keyword">void</span> <span class="identifier">start_receive</span><span class="special">()</span>
  <span class="special">{</span>
</pre>
    <p>
        The function <a class="link" href="../reference/basic_datagram_socket/async_receive_from.html"
                        title="basic_datagram_socket::async_receive_from">ip::udp::socket::async_receive_from()</a>
        will cause the application to listen in the background for a new request.
        When such a request is received, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object will invoke the <code class="computeroutput"><span class="identifier">handle_receive</span><span
            class="special">()</span></code> function with two arguments: a value of
        type <a class="link" href="../reference/error_code.html" title="error_code">error_code</a> indicating
        whether the operation succeeded or failed, and a <code class="computeroutput"><span
            class="identifier">size_t</span></code>
        value <code class="computeroutput"><span class="identifier">bytes_transferred</span></code> specifying
        the number of bytes received.
    </p>
    <pre class="programlisting">    <span class="identifier">socket_</span><span class="special">.</span><span
            class="identifier">async_receive_from</span><span class="special">(</span>
        <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span
                class="special">(</span><span class="identifier">recv_buffer_</span><span
                class="special">),</span> <span class="identifier">remote_endpoint_</span><span class="special">,</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span
                class="special">(&amp;</span><span class="identifier">udp_server</span><span
                class="special">::</span><span class="identifier">handle_receive</span><span
                class="special">,</span> <span class="keyword">this</span><span class="special">,</span>
          <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">placeholders</span><span class="special">::</span><span
                class="identifier">error</span><span class="special">,</span>
          <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">placeholders</span><span class="special">::</span><span class="identifier">bytes_transferred</span><span
                class="special">));</span>
  <span class="special">}</span>
</pre>
    <p>
        The function <code class="computeroutput"><span class="identifier">handle_receive</span><span
            class="special">()</span></code> will service the client request.
    </p>
    <pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">handle_receive</span><span
            class="special">(</span><span class="keyword">const</span> <span class="identifier">asio</span><span
            class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span
            class="identifier">error</span><span class="special">,</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="comment">/*bytes_transferred*/</span><span class="special">)</span>
  <span class="special">{</span>
</pre>
    <p>
        The <code class="computeroutput"><span class="identifier">error</span></code> parameter contains
        the result of the asynchronous operation. Since we only provide the 1-byte
        <code class="computeroutput"><span class="identifier">recv_buffer_</span></code> to contain the
        client's request, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object would return an error if the client sent anything larger. We can ignore
        such an error if it comes up.
    </p>
    <pre class="programlisting">    <span class="keyword">if</span> <span class="special">(!</span><span
            class="identifier">error</span><span class="special">)</span>
    <span class="special">{</span>
</pre>
    <p>
        Determine what we are going to send.
    </p>
    <pre class="programlisting">      <span class="identifier">boost</span><span class="special">::</span><span
            class="identifier">shared_ptr</span><span class="special">&lt;</span><span
            class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span
            class="special">&gt;</span> <span class="identifier">message</span><span class="special">(</span>
          <span class="keyword">new</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">string</span><span class="special">(</span><span class="identifier">make_daytime_string</span><span
                class="special">()));</span>
</pre>
    <p>
        We now call <a class="link" href="../reference/basic_datagram_socket/async_send_to.html"
                       title="basic_datagram_socket::async_send_to">ip::udp::socket::async_send_to()</a>
        to serve the data to the client.
    </p>
    <pre class="programlisting">      <span class="identifier">socket_</span><span class="special">.</span><span
            class="identifier">async_send_to</span><span class="special">(</span><span
            class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span
            class="special">(*</span><span class="identifier">message</span><span class="special">),</span> <span
            class="identifier">remote_endpoint_</span><span class="special">,</span>
          <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span
                class="special">(&amp;</span><span class="identifier">udp_server</span><span
                class="special">::</span><span class="identifier">handle_send</span><span class="special">,</span> <span
                class="keyword">this</span><span class="special">,</span> <span class="identifier">message</span><span
                class="special">,</span>
            <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">placeholders</span><span class="special">::</span><span
                class="identifier">error</span><span class="special">,</span>
            <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">placeholders</span><span class="special">::</span><span class="identifier">bytes_transferred</span><span
                class="special">));</span>
</pre>
    <p>
        When initiating the asynchronous operation, and if using <a class="link" href="boost_bind.html"
                                                                    title="boost::bind">boost::bind</a>,
        you must specify only the arguments that match the handler's parameter list.
        In this program, both of the argument placeholders (asio::placeholders::error
        and asio::placeholders::bytes_transferred) could potentially have been removed.
    </p>
    <p>
        Start listening for the next client request.
    </p>
    <pre class="programlisting">      <span class="identifier">start_receive</span><span class="special">();</span>
</pre>
    <p>
        Any further actions for this client request are now the responsibility of
        <code class="computeroutput"><span class="identifier">handle_send</span><span class="special">()</span></code>.
    </p>
    <pre class="programlisting">    <span class="special">}</span>
  <span class="special">}</span>
</pre>
    <p>
        The function <code class="computeroutput"><span class="identifier">handle_send</span><span
            class="special">()</span></code> is invoked after the service request has
        been completed.
    </p>
    <pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">handle_send</span><span
            class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span
            class="identifier">shared_ptr</span><span class="special">&lt;</span><span
            class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span
            class="special">&gt;</span> <span class="comment">/*message*/</span><span class="special">,</span>
      <span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">error_code</span><span class="special">&amp;</span> <span
                class="comment">/*error*/</span><span class="special">,</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="comment">/*bytes_transferred*/</span><span class="special">)</span>
  <span class="special">{</span>
  <span class="special">}</span>

  <span class="identifier">udp</span><span class="special">::</span><span class="identifier">socket</span> <span
                class="identifier">socket_</span><span class="special">;</span>
  <span class="identifier">udp</span><span class="special">::</span><span class="identifier">endpoint</span> <span
                class="identifier">remote_endpoint_</span><span class="special">;</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span><span
                class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span
                class="number">1</span><span class="special">&gt;</span> <span
                class="identifier">recv_buffer_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
    <p>
        See the <a class="link" href="tutdaytime6/src.html" title="Source listing for Daytime.6">full source listing</a>
    </p>
    <p>
        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
    </p>
    <p>
        Previous: <a class="link" href="tutdaytime5.html" title="Daytime.5 - A synchronous UDP daytime server">Daytime.5
        - A synchronous
        UDP daytime server</a>
    </p>
    <p>
        Next: <a class="link" href="tutdaytime7.html" title="Daytime.7 - A combined TCP/UDP asynchronous server">Daytime.7
        - A combined TCP/UDP
        asynchronous server</a>
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                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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